First record of the invasive ascidian Microcosmus squamiger Michaelsen, 1927 (Ascidiacea: Pyuridae) in Jeju Island, South Korea

Ascidians are sessile marine invertebrates and many are invasive. One notorious example is the solitary species Microcosmus squamiger, which has spread from a hypothesized native region in southeastern Australia to temperate waters worldwide and is considered a global marine invader. This study is the first report of M. squamiger from Jeju Island, South Korea. We collected specimens (N = 20) from Unjin and Wimi harbors on Jeju Island and dissected them to characterise their siphons, pharynx, and dorsal tubercle using stereomicroscopy and scanning electron microscopy. The shape of the dorsal tubercle (the peritubercular area filled with two spiral coils) and the length (15-20 mu m) and shape (roof tile or fingernail) of the siphon spine were consistent with previous identifications of the species. Additionally, molecular verification was performed by extracting genomic DNA from four specimens, two each from Unjin and Wimi harbors. By sequencing a section of the mitochondrial cytochrome oxidase subunit I gene and analysing the generated sequences using a maximum likelihood-based phylogenetic tree, we compared to individuals previously identified as M. squamiger and confirmed the existence of two clades. Therefore, the presence of M. squamiger was verified by morphological and phylogenetic identification. Although M. squamiger is currently distributed globally (except in Antarctica), records from India (2006), Okinawa, and Japan (2007 and 2014) suggest that it was introduced into Asia in the last decade. Though no economic or biological damage caused by this species has been reported in South Korea, the high density of M. squamiger (50-60 individuals/rope-meter) is a potential hazard to other native species via fouling and competition for food and space. Therefore, further research is needed to determine the possible vectors and to continue to monitor its possible spread.